According to the CHP report, obtained by TMZ, "For unknown reasons, Mr. Gibson steered his car to the right and struck the rock hillside. He stopped his car at the scene and was contacted by officers from the West Valley CHP Area."

CHP Public Information Officer Leland Tang tells TMZ ... "Gibson told officers in the field he did not know how the car drifted out of the lane and into the hillside." Tang said his officers followed up by asking Gibson if he was on a cell phone and he said no. Tang adds officers also asked Gibson if he had dozed off "but he did not give a firm 'yes' or 'no'." Gibson just repeated several times, "I don't know how the car left the roadway."

The report goes on, "Mr. Gibson supplied all the pertinent information to the officers at the scene, gave a statement, and received a ride home from a friend."

Gibson was not hurt.

According to the report, "DUI is not suspected in the accident."

UPDATE: As for the curious statement in the report ... "For unknown reasons, Mr. Gibson steered his car to the right and struck the rock hillside." Tang tells TMZ his department was in no way trying to insinuate Gibson did it intentionally. Tang says, "It was totally an unintentional act." And Tang says no citation was issued and no charges will be filed.